Tryptophan hydroxylase activity is enhanced under conditions favorable for phosphorylation of proteins. Preliminary kinetic studies revealed that phosphorylating conditions decreased by half the Km of tryptophan hydroxylase for its cofactor 6MPH4. A much smaller change in the Km of tryptophan hydroxylase for its substrate tryptophan was also observed in one experiment. The results of this initial study indicate that the activity of tryptophan hydroxylase like tyrosine hydroxylase, can be affected by cAMP dependent protein kinases. BIBLIOGRAPHIC REFERENCE: Lovenberg, W. and Bruckwick, W.: The enzymology of tryptophan hydroxylase. Symposium on Biochemical and Function of Monoamine Enzymes. Usdin, Earl and Weiner, Norman (Eds.) Marcel Dekker, Inc. In press (1977).